What happens when a coil dies..... copied from an E31 forum:Had an owner have his 1991 Canadian spec 850 towed here as it was running on 6 cylinders. Car has around 230k kms (142k miles) on it. Ultimately discovered the passenger side coil had spilled its guts and self destructed as shown:http://i171.photobucket.com/albums/u...4/IMG_0374.jpg
Which in turn, took out the corresponding DME:http://i171.photobucket.com/albums/u...4/IMG_0375.jpg
Once both replaced, car started on all 12 but on road test, idle wouldn't drop below 1000 rpms and car was surging violently. Replaced EML and all is now better. This is not the first puking coil we have seen up here that has taken out a DME as well but the first to affect the EML. Perhaps a cautionary maintenance replacement of the 25 year old coils may prevent this expensive failure.
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Just as a follow-up. This was not an isolated case - it was the 5th E31 that I have had to do this diagnosis and repair on over the last couple of years. Other 4 were lower mileage J-specs and a US spec 6-speed car. I don't mind as I have plenty of coils and DME's on hand at the moment but I strongly urge you to replace your coils as a maintenance item. Using either OEM Bosch or aftermarket MSD 8207's will provide a stronger spark than your tired, worn out 20-25 year old OEM coils, improving reliability and even fuel economy and... maybe eliminate the additional cost of DME/EML replacement as well.... just my $.05 worth.
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What would you gents say is a good safe interval? From my recent fuel pump conversations it was suggested fuel pumps get replaced approx every 10 yrs as maintenance items. Would you say half that for coils? 5 yrs despite mileage?
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I have replaced at 35,000 - 40,000 miles. That works out to every 18-24 months.
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These pics I got from an E32 owner in Germany from an E32 750 showing similar damages to the ignition coilshttp://produkte.bilder-speicher.de/1...il-defect.htmlhttp://produkte.bilder-speicher.de/1...il-defect.html
But, they are >20 years old, so the quality was good. Why not change the original ignition coils every 10-15 years. I will stick to Bosch.

From Hella Techworld:
INTERNAL SHORT CIRCUITS : Overheating of the coil caused by the aging process, a faulty ignition module, or a faulty output stage in the electronic control unit.
FAULT IN THE VOLTAGE SUPPLY: The coil charging time increases on account of the voltage supply being too low, this can lead to premature wear or overload on the ignition control unit or the output stages in the electronic control unit. This can be caused by faulty wiring or a weak battery.
MECHANICAL DAMAGE: Damage to the ignition cables caused by marten bites. A faulty valve cover gasket and resulting engine oil leaks can damage the insulation of plug slot coils. Both of these causes lead to sparkover, and thus premature wear.
CONTACT FAULT :Contact resistance in the wiring due to humidity penetrating in the primary and secondary area, also frequently caused by engine washing or the use of grit in winter.
3 SYMPTOMS OF A FAULTY IGNITION COIL: SYMPTOMS
•Engine does not start
•Vehicle misfires
•Poor acceleration or loss of power
•Engine control unit switches to limp-home mode
•Engine warning lamp lights up
•Fault code is stored

4 MEASURING THE IGNITION COIL: BASIC PRINCIPLES
Dismantled state
There are different ways of checking the ignition coil:
Testing the resistance values of the coils using the ohmmeter.
Depending on the ignition system and ignition coil design, the following reference values apply: (observe the manufacturer's specifications)

PRACTICAL TIP
Note: If a high-voltage diode is built into an ignition coil to suppress sparks, it is not possible to measure the resistance of the secondary coil. In this case, the following method is helpful:
Connect a voltmeter in series between the secondary winding of the ignition coil and a battery. If the battery is connected in the diode's conducting direction, the voltmeter must display a voltage. After reversing the polarity of the connections in the blocking direction of the diode, no voltage must be displayed. If no voltage is indicated in either direction, it can be assumed that there is an interruption in the secondary circuit. If a voltage is indicated in both directions, the high-voltage diode is faulty

Installed state, the following checks can be used:
Visual inspection
•Check the ignition coil for mechanical damage
•Check the housing for hairline cracks and sealant leaks.
•Check the electrical wiring and plug connections for damage and oxidation.

Check the electrics using a multimeter or oscilloscope
•Check the voltage supply to the ignition coil
•Check the triggering signal from the ignition distributor, ignition control unit, or engine control unit
•Illustration of the high-voltage curve using an oscilloscope or ignition oscilloscope

Testing with the diagnostic unit
•Read out the fault memory of the ignition system or engine control
•Read out parameters

During all testing work on the ignition system, please note that faults established during tests with the oscilloscope are not necessarily faults caused by the electronic system; they can also be caused by a mechanical problem in the engine. This may be the case, for example, if compression is too low in one cylinder, which means the oscilloscope shows the ignition voltage for this cylinder to be lower than that of the other cylinders.

NOTE : Although "diagnosable engine management systems" are installed in today's vehicles, a multimeter or oscilloscope must be used when checking ignition systems. In order to interpret the displayed measuring results and figures correctly, additional employee training is usually required. One important pre-requisite for successful diagnostics is a careful visual inspection at the beginning of the troubleshooting process.